1. Field of the Invention
The present invention relates to an apparatus and method for performing the manipulation of particles suspended in a fluid, using an acoustic standing wave field.
2. State of the Art
When particles suspended in a fluid are subjected to an acoustic standing wave field, the particles displace to the location of the standing wave nodes. The effectiveness of this process varies with the relative densities and compressibilities of the particles and the suspending fluid.
A number of techniques have been proposed, using this phenomenon, to separate particles from a liquid or other fluid. Typically, the fluid is caused to flow through a duct in which an acoustic standing wave field is established, transverse to the length of the duct. The particles accordingly displace to form a series of parallel bands, and then a number of outlet passages are provided to lead the individual bands of particles away from the main flow duct. Because there are engineering difficulties involved in providing a parallel array of narrow outlet passages to collect the particle bands, the tendency is to operate at relatively low frequencies, so that the wavelength of the standing wave field is sufficiently large to provide an adequate spacing between the particle bands (half-wavelength spacing).
The primary acoustic radiation force on a single particle in an acoustic standing wave field is proportional to the operating frequency. Also the distance which a particle needs to move to reach a node decreases with increasing frequency (because the wavelength is smaller and hence the spacing between nodes is smaller). It is therefore easier to concentrate particles (including biological cells) at higher operating frequencies. Ultrasonic cavitation is also less likely to limit the applicable acoustic pressure at higher frequencies. However, the use of higher frequencies, and therefore smaller wavelengths, increases the engineering difficulties involved in providing outlet passages for the individual particle bands. In some cases, instead of separating particles from the suspending fluid, it may be required to form the particles into their bands for the purpose of observation (e.g. for the purpose of an immuno-agglutination assay, as described in GB-2265004). For such cases, it will be appreciated that the particle bands are closer together at the higher frequencies, and therefore even more difficult to observe.
We have now devised an apparatus and method which overcome the difficulties noted above, and can be used whether the particles are to be separated from the suspending fluid or whether they are to be formed into their bands for observation purposes.